Reaction of threonines with aryl nitriles in a steric inversion process



U ited States Patent @fiiee iffiifi the material into alloand threo-series. And the DL- 234 4 9 three-compounds thus obtained are subjected to the conventional optical resolution to separate L-threonine. REACTION OF THRE'CJNENES WITH ARYL As a method for effecting the steric inversion there is TREES IN A STERIQ INVERSION PROCESS 5 known a process which comprises converting allo-cr-amino- Tokio Joyama, 'lloyonaka, .lapan, assignor to Takeda y y acid a miXtl-lfi Containing a Small Pharmaceutical Indu trie Ltd O ak Ja a amount of threo-u-amino-B-hydroxybutric acid) into an u oxazoline carboxylic acid derivative and then hydrolyzing N6 g fi gg zggggr 1957 the product. Also there are known two methods for the 10 preparation of the oxazoline-carboxylic acid derivative. Claims priority, application Japan June 5, 1956 (11 a-Amino-B-hydroxybutric acid is first esterified with hydrogen chloride and alcohol, for example, and 4 Claims (CL 260 307) then reacted with an aryl-imidoether, and the product is hydrolyzed with alkali (D. F. Elliott: J. Chem, Soc, 1949, This invention relates to a method for the steric inver- 15 589; ibid., 1950, 62).

NH ArylC\ CHr-CH-CH-GOOH CHa-CH-OHOOOR OR OH NHz OH NHa-HX OHs-GH-OHOOOR Alkali CH3CH-CHCOOH I I N 0 N C G Aryl Aryl sion of allothreonin an to a roces th e d p S for e preparauon (2) a-Arn1no-,Bnydroxybutyr1c acid is esterifiecl and of threonine and its intermediates b the use of the method.

It is a well known fact that tli reonine is one of the then {leaded Wlth aroylhahde t0 i the Col-r6- essential amino acids and plays an important role as a spondmg, 'aroylammonfi'hydmxybutnc and ester' The product is treated with an acid reagent such as thionylnutrient in daily life as well as in medical treatment.

Threomne is gxtensively distributed in the animal and chloride to form the oxazoline ring and finally hydrolyzed vegetable kingdoms. For example, it is contained in eggs, With alkali Pfister, 3rd at Ell-3 Chem skim-milk, casein and gelatine in the content of about 5.3, 1101 (1949) 4.6, 4.0 and 1.4%, respectively. Threonine, having the It was clarified by studies of the present inventor that structure of u-amino-fl-hydroxybutyric acid, possesses two reaction of a-amino-fi-hydroxybutric acid with an arylasymmetric carbon atoms, and hence it is natural that nitrile in the presence of an alkaline condensing agent there exist tour isomers of this compound. Threonine is readily produces the corresponding 2-aryl-5-methyl-A divided into threoand allo-series according to the conoxazoline-4-carboxylic acid, that when the reaction mixfiguration of the hydroxyl group at the fi-position, and ture is left standing for a long time, it is sometimes pareach of the series is subdivided into D- and L-series actially hydrolyzed to form a-aroylamino-lfi-hydroxybutyric cording to the configuration of the amino group at the acid, and further that hydrolysis of these products afiords a-position. Therefore, a-amino-B-hydroxybutric acid is only threo-u-amino-B-hydroxybutyric acid Whatever steric divided in all into four series, that is, L-threo-, D-threo-, configuration the material may have. L-alloand D-allo-series, and they are called L-threonine, According to the present method the starting material D-threonine, L-allothreonine and D-allothreonine, respeccan be used without being esterified, and therefore the tively. process is much simplified. Moreover, the present method Natural threonine is L-threonine and has an important is difierent from the known methods in the following imsignificance as a nutrient, while the other three isomers portant point. In the known methods not only allo-comdo not exist in the natural world and have no such a pounds but also threo-compounds are inverted because an significance. As in the case of other amino acids, synacid condensing agent is employed, and hence it the ma.- thetic a-aminofl-hydroxybutric acid consists of the four terial contains a small quantity of three-compounds, the isomers in many cases, and therefore if threonine is to product is contaminated with a small amount of allo-coznbe isolated, the product must be subjected to resolution pounds. Consequently, in order to obtain pure reoand (or) inversion. Since synthetic u-amino-fi-hydroxycompounds the product must be further purified over butyric acid generally contains a large amount of DL-allosodium salt (K. Pfister, 3rd et 211.: I. Am. Chem. Soc. 71, isomers besides DL-threo-compounds, it seems more ad 1101 (1949)). Since in the present method only allovantageous for the preparation of L-threonine to invert compounds are inverted, the product is all threo-comthe allo-isomers into the threo-compounds than to resolve pounds even if the material contains three-compounds.

In the present method, it is not necessary to conduct hydrolysis, and when the alkali is removed by an ionexchanger, free 2-aryl-5-methyl-A -oxazoline-4-carboxylic acid is obtained directly. As mentioned before, a-amino- ;S-hydroxybutyric acid is reacted with an arylnitrile in the presence of an alkaline condensing agent in the present method. As the alkaline condensing agent are conveniently used alkali metal alcoholate, etc., and as the arylnitrile, benzonitrile, toluylnitrile, etc. The reaction is generally carried out in a lower aliphatic alcohol, but it may be conducted in another-suitable solvent. When a mixture of the materials is left standing at room temperature for several :days, the reaction proceeds to form the corresponding xazolinecarboxylic acid derivative, but when the reaction mixture is allowed to stand far longer, the oxazoline ring seems to be ruptured by the alkalinity of the reaction mixture toform a-aroyl amino-[i-hydroxybutyric acid. However, under some conditions substantial formation of the oxazolinecarboxylic acid derivative may not be observed. Thus, the reaction mechanism in the present method is not necessarily definite. That is, the product is the pure oxazolinecarboxylic acid derivative, or waroylamino-fi-hydroxybutyric acid, or a mixture of both compounds depending upon the reaction conditions. But any of the products and the mixture can be converted to threonine by acid hydrolysis as will 'be mentioned later.- Therefore,v the kind of the intermediates and the course of the reactions have no important significance as to the objective of the present inventionwhich aims at the steric inversion of threonine. In any case, heating may be applied to shorten the reaction time, but in general drastic reaction conditions are not necessary.

The oxazolinecarboxylic acid derivative or u-aro'ylamino-fl-hydroxybutyric acid or their mixture thus pro-' duced is obtained as an alkaline aqueous solution when the solvent is distilled off, the residue is diluted with water and the unreacted nitrile is recovered. When the alkaline solution is treated with a suitable, slightly acid ionexchanger such as ion-exchange resin, Amberlite-lRC50, to remove the alkali, the oxazolinecarboxylic acid derivative or a-aroylamino-fi-hydroxybutyric acid or their mixture (they are weakly amphoteric electrolytes) is obtained as an aqueous solution. Crystals of the oxazolinecarboxylic acid derivative (a-aryI-S-methyl-A -oxazoline- 4-carboxylic acid) can be readily obtained by concentration of its aqueous solution. If the solution contains a-aroylamino-e-hydroxybutyric acid, it can be separated by shaking the solution with ethyl acetate, butanol or the like. a-aroylarnino-e-hydroxybutyric acid thus obtained can be readily converted to threonine (threo-a-amino-fi hydro'xybutyric acid) by acid hydrolysis. If necessary, the oxazolinecarboxylic acid derivative and a-aroylamino- B-hydroxybutyric acid are resolved into their optically active components by an optically active base or acid directly or after being hydrolyzed.

The course of the reactions in the present invention is shown by the following scheme.

CH5 CH3 l HC -O HCOH Aryl-CN C-Aryl HONH: a

| Ho-N COOH l OOOH (A110) 3 $Hs HOCH Acid HOCH HCIINHC O-Aryl HCNH:

COOH COOH (Three) The oxazoline carboxylic acid derivative and Example I of 2-phenyl-5-methyl-A -oxazoline-4-carboxylic acid isobtained.

Example 2 To a solution of sodium ethoxide prepared from 6.9 g. of metallic sodium and 200 cc. of ethanol are added 35.7 g. of DL-allo-a-amino-fi-hydroxybutyric acid and 70.2 g. of p-toluylnitrile. manner as in Example 1 to obtain 24.6 g. of 2-p-toluyl- S-methyI-A -oxazoline-4-carboxylic acid.

Example 3 To a solution of sodium methoxide prepared from 6.9 g. of metallic sodium and 150 cc. of methanol are added 35.7 g. of DL allo-a-amino-fi-hydroxybutyric acid and 61.8 g. of benzonitrile, and the mixture is allowed to stand at room temperature for 9 days. The solvent is distilled 0E and the residue is left standing for 7 more days. The reaction mixture is diluted with water and the unchanged benzonitrile is taken up in benzene. The aqueous solution is passed through a layer of Amberlite-IRC-SO (Type-H) to remove alkali and evaporated to dryness, and the residue is recrystallized from ethyl acetate to give 19.5 g. of DL-N-benzoylthreonine.

7 Example 4 To 10 g. of the 2-phenyl-5-methyl-A -oxazoline-4-carboxylic acid obtained in Example 1 is added cc. of 15% hydrochloric acid and the mixture is heated on the water bath for 10 hours. After cooling, the separated benzoic acid is filtered afi and the filtrate is passed through a layerof Amberlite IR-4B to remove acid and then concentrated, whereupon 4.2 g. of DL-threonine is obtained.

Example 5 To 32.5 g. of the DL-N-benzoylthreonine obtained in Example 3 is added diluted sulfuric acid prepared from 5 g. of concentrated sulfuricacid and 100 cc. of water, and the mixture is heated on the Water'bath for 8 hours. After cooling, the separated benzoic acid is filtered off, and'the filtrate is shaken with benzene to remove benzoic acid completely. The aqueous solution is neutralized with barium hydroxide solution to remove sulfuric acid ion, treated with decolorizing carbon and concentrated, whereupon 10 g. of DL-threonine is obtained in colorless pillars.-

What is claimed is:

1. A method for the preparation of a member selected from the group consisting of 2-phenyl-5-methyl-A -oxazoline-4-carboxylic acid, 2-p-toluyl-5-methyl-A -oxazoline- 4-carboxylic acid and a mixture of either of these with the corresponding a-aroylamino-fi-hydroxybutyric acid, which comprises condensing a member selected from the group consisting of allothreonine and a mixture of allothre'onine and threonine, with the corresponding member selected from the group consisting of benzom'trile and toluylnitrile in the presence in the reaction medium of an alkali metal lower aliphatic alcoholate.

2. A method for the preparation of 2-phenyl-5-methyl- A oxazoline-4-carboxylic acid, which comprises condensing allothreonine with benzonitrile in the presence in the reaction medium of sodium methoxide.

The mixture is treated in the same 3. A method for the preparation of 2-p-to1uy1-5- threonine with benzonitrile in the presence in the reaction methyl-A oXazoline-4-carboxy1ic acid, which comprises medium of sodium methoxide. condensing allothreonine with p-toluylnitrile in the presence in the reaction medium of sodium ethoxide. References Cited in the file of this P131511t 4. A method for the preparation of 2-pheny1-5-methy1- 5 A -oxazoline-4-carboxylic acid which comprises con- UNITED STATES PATENTS densing a mixture of allothreonine and the corresponding 2,571,940 Pfister et a1 Oct. 16, 1951 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,846,439 August 5, 1958 Tokio J oyama s in the'printed specification It is hereby certified that error appear tion and that the said Letters of the above numbered patent requiring correc Patent should read as corrected below.

31 and 49, for "hydroxybutric", each occ line 11, for "(11" read no Column 2, lines 6, 7, ll, drr'ence, read ==hydro2qbutyric====g column 2 Signed and sealed this 14th day of October 1958,

(SEAL) Attest:

KARL Ha AXLINE Attesting Oflicer ROBERT C. WATSON Commissioner of Patents 

1. A METHOD FOR THE PREPARATION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF 2-PHENYL-5-METHYL-$2-OXAZOLINE-4-CARBOXYLIC ACID, 2-P-TOLUYL-5-METHYL-$2-OXAZOLINE4-CARBOXYLIC ACID AND A MIXTURE OF EITHER OF THESE WITH THE CORRESPONDING A-AROLYAMINO-B-HYDROXYBUTYRIC ACID, WHICH COMPRISES CONDENSING A MEMBER SELECTED FROM THE GROUP CONSISTING OF ALLOTHREONINE AND A MIXTURE OF ALLOTHREONINE AND THREONINE, WITH THE CORRESPONDING MEMBER SELECTED FROM THE GROUP CONSISTING OF BENZONITRILE AND TOLUYLNITRILE IN THE PRESENCE IN THE REACTION MEDIUM OF AN ALKALI METAL LOWER ALIPHATIC ALCOHOLATE. 